The present invention relates to an electric circuit for a camera for controlling exposure and alarming vibrations of the camera.
Conventionally, an electric circuit for a camera for controlling exposure and alarming vibrations of the camera as shown in FIG. 1 is known. The electric circuit is set "vibration alarm mode" when a release button is depressed lightly and contacts 1 and 2 of a switch SW1 are connected and a power source E.sub.B is connected to this electric circuit. In other words, with a trigger switch TRISW on, a voltage divided by a light receiving element Rcds for exposure photometry and by a resistor R1 for comparison is compared with a reference voltage Vref set by a variable resistor VR1 at a comparator COMP and the output from the comparator COMP is applied to a light emitting diode LED for alarming vibrations of camera through a resistor R2. Therefore, the light emitting diode LED is lighted in the dark and extinguished in the light. When the release button is further depressed, the contacts 1, 2 and 3 of the switch SW1 are all connected so that the electric circuit is set at "timer mode." At this time, since the contacts 2 and 3 of the switch SW1 are connected, the resistor R1 is short-circuited, so that an ordinary timer circuit is formed. With the trigger switch TRISW on, a magnet Mg for controlling a shutter and a light emitting diode LED are on. Here, the light emitting diode LED is on, irrespective of the surrounding lightness, which may cause a wrong display. When a mechanical release of the shutter is effected at this time, the shutter blades are opened and, at the same time, the trigger switch TRISW is off, and voltage begins to be integrated by the light receiving element Rcds and an integration condenser C1. When the integrated voltage amounts to the reference voltage Vref, the magnet Mg and the light emitting diode LED are off and the shutter blades are mechanically released and closed, so that a desired shutter time is obtained.
Referring to FIG. 2, there is shown a time chart of the above-mentioned operation. As can be seen from FIG. 2, when the construction of the release button is such that it can be stopped easily within the range in which the above-mentioned wrong display of the light emitting diode LED can be made, the light emitting diode LED is disadvantageously lighted, irrespective of the surrounding lightness when the release button is stopped in the range. Therefore, it is preferable that the stroke of the release button in the range be small as possible. However, when the release mechanism is of a sensor type, the whole stroke is, for example, as small as 1.5 mm, although the normal stroke is in the range of 6 to 8 mm. In this case, it is difficult to adjust the above-mentioned stroke to be sufficiently small. Consequently, it is impossible to obviate the wrong display of the light emitting diode LED.
In order to obviate such wrong display only, the following mechanism could be proposed. Namely, after the mechanical release of the shutter, the switch SW1 is switched to the timer mode. In this case, it is necessary that the magnet Mg be mechanically held before the trigger switch TRISW is off, that is, before the integration is started, and a period of time T1 of about 10 msec will be necessary for the mechanical holding. Referring to FIG. 3, there is shown a time chart of this operation. As can be seen from FIG. 3, in case it is light enough and the magnet Mg and the light emitting diode LED are off at the "alarm mode" and the above-mentioned period of time T1 is less than 10 msec, the magnet Mg is not in the holding condition, which results in that the magnet Mg works improperly. This could be prevented by actuating a mechanical governor until the trigger switch TR1SW is turned on so that the period of time T1 is lengthened up to more than 10 msec. However, the mechanism would become too complex.
Furthermore, some cameras are provided with a synchronous terminals for connecting a strobotron, in which an X-contact is connected to the synchronous terminals and simultaneously with the mechanical release, the strobotron is lighted with the X-contact on. However, in such a system, the strobotron is lighted when the synchronous terminal is connected to the strobotron even if the subject is light enough.
Furthermore, in an electro-shutter type camera having the X-contact, when part of the wiring of the X-contact is employed instead of the wiring of the electro-shutter, a power source line is employed. Namely, referring to FIG. 4, an electric part 1 for the electroshutter is connected to a battery E.sub.B, and a magnet Mg for the electro-shutter is connected between a power source positive line 3 and an output terminal of the electric part 1, and the X-contact 7 is connected between synchronous terminals 5 and 6, with use of part of the power source positive line 3. As to a strobotron, as shown in FIG. 5, the input side of a DC--DC converter 8 is connected to a battery 9, and a diode 10 and a main condenser 11 are connected in series between the output terminals of the DC--DC converter 8. A flash discharge tube 14 and the series circuit comprising a resistor 2 and a trigger condenser 13 are connected in parallel with a main condenser 11. A secondary circuit of a trigger transformer 15 is connected between one terminal of the flash discharge tube 14 and a trigger electrode. Furthermore, there are provided terminals 16 and 17 connected to the synchronous terminals 5 and 6. By the connection of the terminals 16 and 17 to the synchronous terminals 5 and 6, a primary circuit for the trigger condenser 15 is connected in parallel with the trigger condenser 13 through the X-contact 7. Usually, the output of the battery 9 is converted to a high potential by the DC--DC converter 8 and is charged in the main condenser 11 through the diode 10 and, at the same time, in the trigger condenser 13 through a resistor 12. When the terminals 16 and 17 are connected to the synchronous terminals 5 and 6, the X-contact 7 is closed in synchronism with the shutter, whereby the trigger condenser 13 is discharged and the flash discharge tube 14 is triggered so as to be lighted by the discharging of the main condenser 11.
However, when the wiring of the X-contact is performed as mentioned above, normally the voltage charged in the trigger condenser 13 is applied only to a primary side 15.sub.1 of the trigger transformer 15 when the X-contact 7 is closed as shown in FIG. 29. On the other hand, when the X-contact 7 is closed with part G of the power source positive line (employing for wiring the X-contact) disconnected, the charged voltage in the trigger condenser 13 is applied to the electric part 1 through the battery E.sub.B on the side of the camera as shown in FIG. 30. Furthermore, the inner resistance of the battery 2 is so small that most of the charged voltage in the trigger condenser 13 is applied to the electric part 1. Therefore, the electric part 1 is destroyed by application of a high voltage V1 which is beyond an endurable voltage as shown by solid line in FIG. 28. This is apt to occur particularly when it is necessary to separate the electric part 1 from the magnet Mg and the X-contact 7 and others. In other words, when the electric part 1 and the magnet Mg and the X-contact 7 are incorporated in one unit by connecting them by a connector, or when the magnet, the X-contact and the electric circuit are connected by contacts as in a mirror-tube-retractable type camera, since the power source positive line G is connected to the contacts, improper contact is apt to occur.